1. Field of the Invention
This invention relates to a light valve system and method for spatially modulating a readout beam in response to an applied input signal, and more particularly to light valves in which the readout beam is coordinated with the operation of the light valve to enhance its performance.
2. Description of the Related Art
Light valves, generally employing liquid crystals as an electro-optic medium, are used to spatially modulate a readout beam in accordance with an input signal pattern applied to the light valve. They can be used to greatly amplify the input pattern by controlling a readout beam of much greater intensity, to convert spatially modulated incoherent input radiation to a coherent readout laser beam with a similar spatial modulation, for optical data processing, wavelength conversion, or for other purposes that involve the conversion of an input signal pattern to a corresponding spatial modulation on a separate readout beam. The input signal may take the form of a spatially modulated optical input beam, a voltage pattern applied to a CCD array on the light valve, or a scanning electron beam; other means of inputs might also be visualized.
The main parameters of light valves are the input sensitivity, output, and resolution modulation (contrast ratio), as well as output uniformity and frame rate. While high contrast, moderate brightness and color capability are required for command and control displays, very high brightness and resolution, as well as fast response, are required for flight-simulation applications. Optical data processing applications require low wavefront distortion (output uniformity) and high diffraction efficiency. In addition, for real-time portable scene correlators, high frame rate, wide spectral range, small size, and low power consumption are also required. Most of these requirements are met by a cadmium sulfide liquid crystal light valve developed by Hughes Aircraft Company. This device is described in articles by J. Grinberg, A. Jacobson, W. P. Bleha, L. Miller, L. Fraas, D. Boswell and G. Myer, "A New Real-Time Non-Coherent to Coherent Light Image Converter--The Hybrid Field Effect Liquid Crystal Light Valve", Optical Engineering 14, 217 (1975), and J. Grinberg, W. P. Bleha, A. Jacobson, A. M. Lackner, G. Myer, L. Miller, J. Margerum, L. Fraas and D. Boswell, "Photoactivated Birefringent Light-Crystal Light Valve for Color Symbology Display", IEEE Transactions Electronic Devices ED-22, 775 (1975).
The main drawback of the CdS-based light valve has been its slow response time. A second generation, silicon-based liquid crystal light valve has been developed which retains the advantages of the CdS-based light valve and has a considerably faster response time. The silicon-based device is described in an article by U. Efron, J. Grinberg, P. 0. Braatz, M. J. Little, P. G. Reif and R. N. Schwartz, "The Silicon-Liquid Crystal Light Valve", Journal of Applied Physics 57(4) 1356-68 (1985). This article also summarizes some of the prior light valve efforts.
A simplified block diagram of a typical photoactivated light valve system is illustrated in FIG. 1. An input beam 2 is developed from a source such as the screen of a cathode ray tube 4 and imaged through lens 6 onto the input side of a light valve 8. On the other side of the light valve a readout beam 10 is generated by a laser 12, and directed onto the readout side of the light valve by a polarizing beam splitter 14. The input beam 2 establishes a spatial polarization of a liquid crystal layer within the light valve 8, and this layer controls the retro-reflection of the readout beam from the light valve. The portions of the readout beam which are incident upon locations in the liquid crystal layer wherein the liquid crystal molecules have been rotated in response to the voltage generated by the input radiation are retro-reflected back through beam splitter 14 to emerge as an out beam 16. In this example, the liquid crystal in the light valve modulates the spatial intensity of the readout beam into a corresponding but amplified intensity pattern of the input beam.
The internal construction of a silicon-based liquid crystal light valve which is suitable for this purpose is shown in FIG. 2. An input image beam on the right hand side of the device is identified by reference numeral 18, while a readout beam 20 is directed onto, and reflected from, the left hand side of the device. A layer of high resistivity silicon photoconductor 22 has a thin p. back contact layer 24 formed on its input side. This back contact provides a high sheet conductivity to present a very small load at any point in the device's cross-section where carriers are generated. It also attains a linear operation of the device, avoiding a situation in which the sensitivity and resolution are dependent upon the input light level, and also provides higher output uniformity under dark conditions. An SiO: oxide layer 26 is provided on the input side of back contact 24, with a fiber optic plate 28 adhered to the oxide layer by means of an optical cement 30. A DC-biased n-type diode guard ring 32 is implanted at the opposite edge of the silicon photoconductor wafer 22 from back contact 24 to prevent peripheral minority carrier injection into the active region of the device. An SiO.sub.2 gate insulator layer 34 is formed on the readout side of the silicon photoconductor wafer 22. Isolated potential wells are created at the Si/SiO.sub.2 interface by means of an n-type microdiode array 36. This prevents the lateral spread of signal electrons residing at the interface.
A unified thin film dielectric mirror 38 is located on the readout side of the gate oxide layer 34 to provide broad-band reflectivity, as well as optical isolation to block the high intensity readout beam from the photoconductor. A thin film of fast response liquid crystal 40 is employed as the light modulating electro-optic layer on the readout side of mirror 38. A front glass plate 42 is coated with an indium tin oxide (ITO) counterelectrode 44 adjacent the liquid crystal. The front of glass plate 40 is coated with an anti-reflection coating 46, and the whole structure is assembled within an airtight anodized aluminum holder.
Silicon photoconductor 22 is coupled with oxide layer 34 and transparent metallic electrode coating 44 to form an MOS structure. The combination of the insulating liquid crystal, oxide and mirror act as the insulating gate of the MOS structure.
In operation, an alternating voltage source 48 is connected on one side to back contact 24 by means of an aluminum back contact pad 50, and on its opposite side to counterelectrode 44. The voltage across the two electrodes causes the MOS structure to operate in alternate depletion (active) and accumulation (inactive) phases. In the depletion phase, the high resistivity silicon photoconductive layer 22 is depleted and electron-hole pairs generated by input light beam 18 are swept by the electric field in the photoconductor, thereby producing a signal current that activates the liquid crystal. The electric field existing in the depletion region acts to sweep the signal charges from the input side to the readout side, and thus preserve the spatial resolution of the input image. The polarized readout beam 20 enters the readout side of the light valve through glass layer 42, passes through the liquid crystal layer, and is retro-reflected by dielectric mirror 38 back through the liquid crystal. Since the conductivity of each pixel in photoconductive layer 22 varies with the intensity of input beam 18 at that pixel, a voltage divider effect results which varies the voltage across the corresponding pixel of the liquid crystal in accordance with the spatial intensity of the input light. As is well known, the liquid crystals at any location will orient themselves in accordance with the impressed voltage, and the liquid crystal orientation relative to the readout light polarization at any particular location will determine the amount of readout light that will be reflected back off the light valve at that location. Thus, the spatial intensity pattern of the input light is transferred to a spatial liquid crystal orientation pattern in the liquid crystal layer, which in turn controls the spatial reflectivity of the light valve to the readout beam.
One branch in the development of liquid crystal light valves is the charge-coupled device light valve (CCD-LCLV). This type of device has applications mainly in optical data processing as an electronically addressed optical light modulator for spectrum analysis, image correction, radar, and spread-spectrum signal processing. The general structure and operation of such devices is described in an article by Uzi Efron et al., "Silicon Liquid Crystal Light Valves: Status and Issues:", Optical Engineering, Vol. 22, No. 6, Nov./Dec. 1983, pages 682-686.
For both photoactivated and CCD addressed light valves, an important function of the dielectric mirror 38 is to block readout light and prevent it from activating the photoconductor substrate 22. The intensity of the readout beam may be in the order of 10.sup.6 -10.sup.8 times the input beam intensity. During the active (depletion) phase of light valve operation, minority carriers are transported from the back face of the photoconductor layer to the readout face adjacent the dielectric mirror. It is this accumulation of a small quantity of spatially resolved carriers at the readout face that produces a voltage pattern for activating the liquid crystal layer. Since the photoconductor layer 22 is photosensitive, a dielectric mirror/light blocking layer 38 is required that will prevent the high intensity readout light from generating spatially unresolved carriers in the photoconductor that would otherwise swamp the signal charge. Typically, the dielectric mirror/light blocking layer 38 must attenuate the readout beam by a factor of about 10.sup.6 or larger, so that the number of carriers accumulated during the active phase due to light leakage through the dielectric mirror/light blocking layer does not approach or exceed the signal charge. It is quite difficult to fabricate a dielectric mirror with this capability. Although an attenuation of 10.sup.7 has been achieved, some applications require greater attenuations, for which adequate dielectric mirrors are not presently available.
As a possible substitute for a dielectric mirror, a recently developed metal matrix mirror has been demonstrated to provide excellent electrical and optical properties for valves operating in the infrared region. This type of mirror is described in the co-pending U.S. patent application Ser. No. 759,004, "Reflective Matrix Mirror Visible to Infrared Convertor Light Valve" by P. O. Braatz, and assigned to Hughes Aircraft Company.
A metal matrix mirror is illustrated in FIG. 3. A matrix of reflective islands 52 is formed on an insulative layer 54 such as SiO.sub.2. The islands 52 are separated from each other so as to avoid short-circuits across the face of the mirror. The dimensions of the individual islands 52 are determined from a minimum size for adequate reflection, on the order of 5-20 microns, and the resolution or pixel element size for which the light valve is designed. The thickness of the islands depends upon the specific reflective material employed. There is a basic requirement that the free electron density of the reflective material be sufficient to interact with the readout radiation and scatter it back out of the material. Metals such as aluminum or silver or metal/semiconductor compounds such as platinum-silicide may be used.
The useful range of light valves employing metal matrix mirrors has been limited principally to infrared radiation because of the bandgap of the silicon substrate. In the visible region, visible readout light leaks through the vacant channels separating the metal islands, causing activation of the underlying photoconductor. Since only about 70% of the readout surface is occupied by the reflective islands 52, enough light leaks through between the islands to effectively prevent operation in the visible region. Thus, significant limitations are encountered with both dielectric and metal matrix mirrors.